I think there's an easy way to answer your question... all you have to do is ask yourself what is the end result of anything you do in Blend?
The answer is XAML. So if this tool generates XAML for you, why wouldn't you work directly with XAML in the first place? XAML is human readable, it's easy to understand and master and between VS Intellisense and Resharper code completion there's really not a whole lot of typing required.
The time you invest in mastering Blend is much better invested in mastering XAML because Blend is just a tool used to generate XAML in a visual way so working directly with XAML means better understading of the underlying technology and taking full control of your code instead of relying on some black box tool to generate something you don't really understand.
And what if you invest all that time in mastering Blend and you end up in some other company 6 months down the road and they decided not to use Blend? If you don't know XAML, you'll now have to invest more time to learn it and if you do know XAML you'll be good to go in any environment as I really doubt it any company would force coders who need to work with UI as well to use Blend and not allow them to work with XAML directly.
There are a few scenarios Blend would save you quite a bit of time (people mentioned visual states and animations) so it's certainly good to have Blend on such occasions but if you're developing typicall LOB apps there's pretty much nothing there that will make you feel like you really, really need Blend...